Transmitter architecture and related methods

ABSTRACT

A radio frequency (RF) transmitter includes one or more power amplifiers and a controller that is configured to adjust amplitudes and phases of RF input signals of the one or more power amplifiers and supply voltages applied to the one or more power amplifiers. In embodiments where multiple power amplifiers are used, a combiner may be provided to combine outputs of the power amplifiers. In at least one implementation, amplitude adjustment of the RF input signals of the one or more power amplifiers may be used to provide transmit power control and/or power backoff for the RF transmitter.

FIELD

Subject matter disclosed herein relates generally to radio frequency(RF) circuits and, more particularly, to devices, systems, andtechniques for use in implementing RF transmitters that are capable ofachieving high linearity and high efficiency simultaneously.

BACKGROUND

As is known in the art, a radio frequency (RF) transmitter is a devicethat produces RF signals. RF transmitters may be included, for example,as part of a radio communication system that uses electromagnetic waves(radio waves) to transport information over a distance.

As is also known, a trade-off must generally be made in RFcommunications transmitters between energy efficiency and linearity.Over the decades of development of the RF transmitter, it has generallybeen true that one could obtain either high efficiency or highlinearity, but not both. It would, therefore, be desirable to providesystems and techniques that avow a user to transmit data carrying RFsignals with both high efficiency and high linearity.

SUMMARY

Systems and techniques are described herein that allow data carryingradio frequency (RF) signals to be transmitted with both high efficiencyand high linearity. Digital control may be maintained over both theamplitudes and the phases of RF input signals applied to one or morepower amplifiers within an RF transmitter. Digital control may also bemaintained over the supply voltages applied to the one or more poweramplifiers. In some embodiments, non-linear power amplifiers may be usedto achieve high power efficiency within the RF transmitter. Digitalcontrol techniques may be used to achieve linearity and to furtherenhance efficiency. In at least one implementation, the amplitudes ofone or more RF input signals of the one or more power amplifiers may becontrolled to achieve back off in the power amplifier to generate adesired output power level.

In accordance with one aspect of the concepts, systems, circuits, andtechniques described herein, an RF transmitter comprises: a first RFpower amplifier; a second RF power amplifier; a combiner to combineoutput signals of the first and second RF power amplifiers; a firstdigital-to-RF modulator to generate an RF input signal for the first RFpower amplifier based on first information indicative of a firstamplitude and a first phase; a second digital-to-RF modulator togenerate an RF input signal for the second RF power amplifier based onsecond information indicative of a second amplitude and a second phase;a first voltage control unit to provide a variable supply voltage to thefirst RF power amplifier in response to a first control signed; a secondvoltage control unit to provide a variable supply voltage to the secondRF power amplifier in response to a second control signal; and acontroller to provide the first information, the second information, thefirst control signal, and the second control signal based, at least inpart, on transmit data to be transmitted from the RF transmitter,wherein the controller controls at least one of the first amplitudeassociated with the first information and the second amplitudeassociated with the second information to set an output power level ofthe RF transmitter.

In accordance with another aspect of the concepts, systems, circuits,and techniques described herein, a machine implemented method foroperating an RF transmitter having at least two digital-to-RF modulatorsdriving at least two corresponding power amplifiers and a combiner tocombine output signals of the at least two power amplifiers comprises:obtaining transmit data to be transmitted from the RF transmitter;generating input information for the at least two digital-to-RFmodulators based, at least in part, on the transmit data, the inputinformation to control amplitudes and phases of RF output signals of theat least two digital-to-RF modulators, wherein generating inputinformation includes controlling one or more amplitudes associated withthe at least two digital-to-RF modulators to set an output power levelof the RF transmitter; delivering the output signals of the at least twodigital-to-RF modulators to corresponding inputs of the at least twopower amplifiers; selecting supply voltages for each of the at least twopower amplifiers based, at least in part, on the transmit data; andcombining the output signals of the at least two power amplifiers togenerate a transmitter output signal.

In accordance with a still further aspect of the concepts, systems,circuits, and techniques described herein, an apparatus is provided thatcomprises a computer readable medium having instructions stored thereonthat, when executed by a computer, perform a method for operating an RFtransmitter having at least two digital-to-radio frequency (RF)modulators feeding at least two power amplifiers and a combiner tocombine output signals of the at least two power amplifiers, the methodcomprising: obtaining transmit data to be transmitted from the RFtransmitter; providing input information for the at least twodigital-to-RF modulators based, at least in part, on the transmit data,the input information to control amplitudes and phases of RF outputsignals of the at least two digital-to-RF modulators, wherein generatinginput information includes controlling one or more amplitudes associatedwith the at least two digital-to-RF modulators to set an output powerlevel of the RF transmitter; and selecting supply voltages for each ofthe at least two power amplifiers based, at least in part, on thetransmit data.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features may be more fully understood from the followingdescription of the drawings in which:

FIG. 1 is a block diagram illustrating an exemplary radio frequency (RF)transmitter in accordance with an embodiment;

FIG. 2 is a block diagram illustrating an exemplary RF transmitter inaccordance with another embodiment;

FIG. 3 is a block diagram illustrating an exemplary RF transmitterhaving at least four power amplifiers in accordance with an embodiment;

FIG. 4 is a block diagram illustrating an exemplary RF transmitterhaving a single power amplifier in accordance with an embodiment;

FIG. 5 is a block diagram illustrating an exemplary RF transmitter inaccordance with still another embodiment;

FIG. 6 is a schematic diagram illustrating an exemplary voltage controlunit in accordance with an embodiment;

FIG. 7 is a schematic diagram illustrating an exemplary voltage controlunit in accordance with another embodiment;

FIG. 8 is a flow diagram illustrating a method for operating an RFtransmitter having at least two digital-to-RF modulators driving atleast two power amplifiers in accordance with an embodiment;

FIG. 9 is a flow diagram illustrating a method for operating an RFtransmitter having a single digital-to-RF modulator driving a singlepower amplifier in accordance with an embodiment;

FIG. 10 is a diagram illustrating an exemplary sample window that may beused to make a voltage level change decision in a power amplificationsystem in accordance with an embodiment; and

FIG. 11 is a flow diagram illustrating a method for selecting voltagelevels for one or more power amplifiers of a power amplification systembased on a sample window in accordance with an embodiment;

FIG. 12 is a plot illustrating the efficiency and output power of apower amplification system having a single power amplifier as a functionof normalized RF chive power at two different dc power supply levels;

FIG. 13 is a pair of IQ plots illustrating achievable RF outputamplitudes for a power amplification system having a single poweramplifier using two different supply levels;

FIG. 14 is a plot of efficiency versus normalized output power for apower amplification system having a single power amplifier using twodifferent supply levels in accordance with an embodiment;

FIG. 15 is an IQ plot illustrating achievable output signal ranges for apower amplification system having two power amplifiers and a combiner,where each power amplifier is supplied from one of two supply levels;

FIG. 16 is an IQ plot illustrating operation of an exemplary AsymmetricMultilevel Outphasing (AMO) based amplification system that includestwo-power amplifiers and two power supply levels;

FIG. 17 is an IQ plot illustrating a control technique that may be usedwith a power amplification system having two power amplifiers and acombiner to operate the power amplifiers at or close to their achievablemaximum amplitudes, while minimizing output at an isolation port of thecombiner; and

FIG. 18 is a series of plots illustrating how power amplifier drivelevels and supply levels may be adjusted to control output power over anoutput power range in accordance with an embodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an exemplary radio frequency (RF)transmitter 10 in accordance with an embodiment. As will be described ingreater detail, RF transmitter 10 is capable of simultaneously achievingboth high efficiency and high linearity. As illustrated, RF transmitter10 may include: a controller 12; first and second digital to RFmodulators 14, 16; first and second power amplifiers 18, 20; first andsecond voltage control units 22, 24; a power combiner 26; and an energyrecovery module 30. RF transmitter 10 may be coupled to one or moreantennas 32 and/or other transducers to facilitate the transmission ofRF signals to one or more remote wireless entities. In someimplementations, first and second power amplifiers 18, 20 may usesubstantially the same amplifier design. In other implementations,different amplifier designs and/or architectures may be used. In someembodiments, first and second power amplifiers 18, 20 may be non-linearamplifiers (and, in some cases, highly non-linear amplifiers). As iswell known, non-linear amplifiers generally operate more efficientlythan linear amplifiers.

First and second digital to RF modulators 14, 16 are operative forgenerating RF input signals for first and second power amplifiers 18,20, respectively, based on information received from controller 12.First and second voltage control units 22, 24 are operative forproviding variable supply voltages to first and second power amplifiers18, 20, respectively, based on control signals received from controller12. In a typical implementation, controller 12 will receive or otherwiseobtain a stream of data to be transmitted from RF transmitter 10 (i.e.,transmit data). Controller 12 may then use this transmit data, amongother things, to provide signals for first and second digital to RFmodulators 14, 16 and first and second voltage control units 22, 24 thatwill result in the transmission of the transmit data from antenna(s) 32.Controller 12 may update the information delivered to first and seconddigital to RF modulators 14, 16 and the control signals delivered tofirst and second voltage control units 22, 24 on a sample-by-samplebasis in some implementations.

In at least one implementation, controller 12 can provide independentcontrol to each of first and second voltage control units 22, 24 so thatdifferent supply voltages can be simultaneously applied to first andsecond power amplifiers 18, 20. Likewise, in some implementations,controller 12 can provide different input information to first andsecond digital to RF modulators 14, 16 so that different RF inputsignals are simultaneously applied to first and second power amplifiers18, 20. Power combiner 26 is operative for combining the output signalsof first and second power amplifiers 18, 20 to generate an RF transmitsignal at an output thereof. The RF transmit signal may then bedelivered to antenna(s) 32 for transmission into a wireless channel. Aswill be appreciated, the RF transmit signal should include an accuraterepresentation of the original transmit data.

First and second power amplifiers 18, 20 may each include any type ofpower amplifier capable of amplifying an RF signal. In someimplementations, first and second power amplifiers 18, 20 may benon-linear amplifiers to improve the efficiency of operation of the RFtransmitter. In some embodiments, highly non-linear amplifiers may beused. First and second power amplifiers 18, 20 may use the sameamplifier design or different amplifier designs. Likewise, first andsecond power amplifiers 18, 20 may use the same amplifier architectureor different amplifier architectures. First and second voltage controlunits 22, 24 may each include any type of circuit, component, or systemthat is capable of controllably varying a supply voltage level appliedto a power amplifier in an RF transmitter. These units may include, forexample, variable power supplies, discrete power supplies, batteries,multi-level power converters, and/or switching circuits that are capableof switching between preset voltage potentials.

First and second digital to RF modulators 14, 16 may include any type ofcircuits or components that are capable of converting digital inputinformation representative of a time varying amplitude and a timevarying phase into an analog RF output signal having correspondingamplitude and phase characteristics. Power combiner 26 may include anytype of device or structure that is capable of combining multiple RFsignals. This may include, for example, a hybrid combiner, a transformercombiner, a Wilkinson combiner, and or others. Power combiner 26 may bean isolating combiner or a non-isolating combiner.

As described above, controller 12 may receive or otherwise acquiretransmit data that needs to be transmitted into a wireless channel. Thetransmit data may be in any format (e.g., a binary bit stream; I and Qdata; etc.). Controller 12 may then use this data, as well as otherpossible factors, to provide signals for first and second digital to RFmodulators 14, 16 and first and second voltage control units 22, 24. Insome implementations, the goal may be to generate an RF transmit signalthat includes an accurate representation of the transmit data. Any of anumber of different modulation and coding schemes (MCSs) may be used torepresent the transmit data within the RF transmit signal. The MCS mayinclude, for example, binary phase shift keying (BPSK), quadrature phaseshift keying (QPSK), quadrature amplitude modulation (e.g., QAM, 16 QAM,64 QAM, 128 QAM, etc), orthogonal frequency division multiplexing(OFDM), and/or others. Some of these MCSs have relatively high peak toaverage power ratios. As is well known, MCSs having high peak to averagepower ratios typically require highly linear power amplification toprovide an accurate representation of transmit data. In variousembodiments described herein, transmission systems and techniques aredescribed that are capable of providing efficient power amplificationwith sufficient linearity to support MCSs having high peak to averagepower ratios.

As shown in FIG. 1, after controller 12 acquires transmit data, it mayuse the data to provide input information to first and second digital toRF modulators 14, 16. In one possible approach, controller 12 mayprovide separate I and Q data to each of the first and second digital toRF modulators 14, 16. That is, controller 12 may generate I₁, Q₁ forfirst digital to RF modulator 14 and I₂, Q₂ for second digital to RFmodulator 16. First and second digital to RF modulators 14, 16 may thenuse the I, Q information to modulate an RF carrier wave to generate acorresponding RF signal at an output thereof. As is well known, I and Qdata is generally representative of an amplitude and a phase. Thus, I₁and Q₁ may, for example, have a corresponding amplitude A₁ and phase θ₁.The RF signal output by first digital to RF modulator 14 in response toI₁ and Q₁ may therefore be an RF signal having amplitude A₁ and phaseθ₁. In some implementations, the input information provided to first andsecond digital to RF modulators 14, 16 may be in a format other than Iand Q. For example, in one possible approach, amplitude (A1, A2) andphase (θ₁, θ₂) information may be delivered to first and second digitalto RF modulators 14, 16 by controller 12. As described above, the inputinformation applied to first and second digital to RF modulators 14, 16may change on a sample by sample basis in some embodiments.

In some implementations, first and second voltage control units 22, 24may each be capable of providing one of a plurality of predeterminedvoltages to corresponding power amplifiers 18, 20 in response to controlsignals from controller 12. Thus, a control signal V_(CONT1) may selecta voltage value for power amplifier 18 and a control signal V_(CONT2)may select a voltage value for power amplifier 20. As with the inputinformation applied to first and second digital to RF modulators 14, 16,the supply voltage values applied to first and second power amplifiers18, 20 may change on a sample by sample basis in some embodiments.

In addition to the above, in some embodiments, controller 12 may use theamplitude information delivered to first and second digital to RFmodulators 14, 16 (e.g., the amplitude value associated with I₁ and Q₁,etc.) to control/adjust a power level output by the RF transmitter 10(e.g., a transmit power level). For example, controller 12 may use areduced amplitude value for one or both of the digital to RF modulators14, 16 when a lower transmit power level is desired.

In some embodiments, controller 12 may determine input information forfirst and second digital to RF modulators 14, 16 and control informationfor first and second voltage control units 22, 24 in real time based onthe transmit data. In some other embodiments, an optional lookup table(LUT) 36 may be used to provide the required information. Controller 12may retrieve values from LUT 36 for the first and second digital to RFmodulators 14, 16 and the first and second voltage control units 22, 24that are designed to accurately represent the transmit data in thetransmit signal.

In some embodiments, power combiner 26 may be an isolating combinerhaving an isolation port. As is well know, an isolating combiner willsometimes output energy at the isolation port due to, for example,mismatches, imbalances, and/or reflections in the circuitry coupled tothe combiner. Typically, a resistive termination will be coupled to theisolation port of an isolating combiner to provide an impedance matchfor the port and to dissipate any energy output from the port. In someembodiments, an energy recovery module 30 may be coupled to theisolation port of an isolating combiner, rather than a conventionalresistive termination, for use in recovering some or all of the energythat would otherwise have been dissipated. Energy recovery module 30 mayinclude circuitry for converting the recovered energy into a usefulform. For example, the recovered energy may be converted to a form thatcan be used to charge a battery. Alternatively, the recovered energy maybe converted to a form that may be used to energize other circuitrywithin RF transmitter 10.

In the embodiment illustrated in FIG. 1, separate voltage control units22, 24 are provided for first and second power amplifiers 18, 20. Insome implementations, the functions of multiple voltage control unitsmay be implemented using a single voltage control structure. FIG. 2 is ablock diagram illustrating an exemplary radio frequency (RF) transmitter40 in accordance with an embodiment. As shown in FIG. 2, a singlevoltage control unit 34 may be used to provide variable voltages forboth of power amplifiers 18, 20.

In some embodiments, power amplification systems are provided thatincorporate dynamic selection from among a discrete set of voltagelevels for providing drain bias voltages to a set of power amplifiers(PAs). Selection from among multiple discrete voltage levels in settingone or more drain bias voltages is common to a number of systemarchitectures. This includes systems that select from among a discreteset of input voltages and then provide additional regulation to providea continuously-varying drain voltage (see, e.g., “Multilevel PowerSupply for High Efficiency RF Amplifiers,” by Vasic et al., 2009 IEEEApplied Power Electronics Conference, pp. 1233-1238, February 2009; andU.S. Pat. No. 7,482,889 to Wilson, entitled “High EfficiencyAmplification.”) and systems that directly exploit discrete drainlevels, including “class G” amplifiers (see, e.g., “Average Efficiencyof Class-G Power Amplifiers,” by F. H. Raab, IEEE Transactions onConsumer Electronics, Vol. CE-32, no, 2, pp. 145-150, May 1986; and “AClass-G Supply Modulator and Class-E PA in 130 nm CMOS,” by Walling etal., IEEE Journal of Solid-State Circuits, Vol. 44, No. 9, pp.2339-2347, September 2009), multi-level LINO (MLINC) Power Amplifiers(see, e.g., U.S. Patent Application Publication 2008/0019459 to Chen etal. entitled “Multilevel LINC Transmitter;” and U.S. Patent ApplicationPublication US 2010/0073084 to Hur et al. entitled “Systems and Methodsfor a Level-Shifting High-Efficiency LINC Amplifier using Dynamic PowerSupply” and Asymmetric Multilevel Outphasing (AMO) Power Amplifiers(see, e.g., U.S. Pat. Nos. 8,026,763 and 8,164,384 to Dawson et al.entitled “Asymmetric Multilevel Outphasing Architecture for RFAmplifiers” and “A 2.4-GHz, 27-dBm Asymmetric Multilevel OutphasingPower Amplifier in 65-nm CMOS” by Godoy at al., IEEE Journal ofSolid-State Circuits, 2012), in the present application, dynamicselection from among a discrete set of voltage levels (or “levelselection”) is primarily discussed in the context of a single ormulti-amplifier, multi-level power amplifier architecture (such as, forexample, the power amplifier architecture used in RF transmitter 10 ofFIG. 1). However, the level selection techniques discussed herein canalso be used to improve performance in other architectures utilizingdiscrete voltage levels, including those enumerated above.

Level selection can be performed based upon the instantaneous outputenvelope value or, when processing digital data from which to synthesizethe RF output, upon an individual digital sample to be synthesized. Inone possible approach, the lowest discrete voltage level or levels maybe used that are compatible with synthesizing the required instantaneousRF output in order to provide the highest instantaneous “drain”efficiency. In at least one embodiment, discrete voltage levels may beselected based upon a “window” of data to be transmitted, rather than asingle sample or “Instantaneous” value. In this manner, the number oflevel transitions that are used to synthesize a particular data streamcan be reduced. In many cases, this technique may result in highervoltage levels being used than are absolutely necessary to synthesize aparticular sample (thus reducing power amplifier “drain” efficiency).However, overall efficiency can be improved (or at least its degradationreduced) by mitigating the energy cost of transitioning among levels.Moreover, linearity (e.g., as measured by adjacent channel leakage ratio(ACLR) or adjacent channel power ratio (ACPR)), error vector magnitude(EVM), and other waveform quality metrics can be improved by reducingthe number of level transitions required. This is because the systemdisturbance caused by transitioning levels on one or more PAs willtypically have a cost in terms of the output trajectory, even thoughthis can be compensated for using digital pre-distortion or othertechniques.

In some embodiments, decisions may be made about the discrete voltagelevels to apply based on one or more extended time periods, rather thanrelying solely on instantaneous conditions. In one approach, forexample, a decision as to whether or not to change selected level(s)looks across a window of samples N_(W) long (i.e., from a current sampleunder consideration). As will be described in greater detail, FIG. 10 isa diagram illustrating such a sample window 220 and FIG. 11 is aflowchart illustrating an exemplary method 230 for selecting voltagelevels using a sample window. These samples may be represented as I,Qdata values, as amplitudes/phases of the desired outputs, as vectorvalues to be synthesized, etc., or may simply represent the output poweror voltage amplitudes desired. For the current sample 222, the systemmay only transition to a lower level or set of levels (i.e., providing alower maximum power) if that lower level or set of levels can providesufficient output power over the whole window 220 of N_(W) samples. Ifmore power delivery is needed for the current sample 222 than can beprovided by the level/set of levels used in the previous sample 224,then a transition is made to a level/set of levels capable of providingsufficient power. Otherwise, the level/set of levels remains unchangedfrom the previous sample 224 to the current sample 222.

Using the above-described technique reduces the number of transitions byensuring that a “downward” level transition only occurs if an “upward”transition won't be necessary again in the near future. At the sametime, an “upward” transition will always be used if needed to supportthe required output power during a given sample. It will be appreciatedthat many variations are possible which provide further reduction innumber of transitions. For example, in one alternative approach, adecision may be made to make either an “upward” or a “downward”transition as long as it is certain that another transition will not beneeded for at least a minimum number of samples (i.e., within a windowof samples). In another alternative approach, if an “upward” transitionis needed to support an output for less than a certain number ofsamples, that transition could be eliminated, and the power amplifiersdriven to provide an output as close as possible to the desired output(during either the sample in question or averaged in some manner overmultiple samples). This may reduce transitions, at the expense of somewaveform quality degradation. If any of the above-described techniquesare used, the input drives for the individual power amplifiers may beadjusted accordingly for the selected drain voltage levels in order toprovide the desired total output.

In some implementations, the above-described technique may be combinedwith other techniques including, for example, peak-to-average powerratio (PAPR) reduction techniques, techniques using hysteresis in thethresholds used to make level selection determinations, techniques thatfilter the data before and/or after the level-selection process, and/orothers. In some implementations, the window lengths that are used withthe above-described techniques may be dynamically and/or adaptivelyselected based on a performance criterion. In one approach, for example,the window lengths may be selected to maximize a combination ofefficiency and output quality metrics.

To provide a better understanding of the operation and benefits of theamplifier architectures described herein, the performance of a singlepower amplifier (PA) having two different do supply voltages will now bedescribed. This power amplifier could represent any number of PA classes(e.g., A,AB,B,F, inverse F, etc.) and could also represent a compositeamplifier comprising an aggregation of output from multiple smaller PAsoperated together (e.g., a Doherty amplifier, outphasing amplifier,etc.) FIG. 12 is a plot showing the efficiency and (normalized) outputpower of a power amplifier as a function of normalized RF drive power attwo different do power supply levels (La, a lower do supply, level 1,and a higher do supply, level 2). It can be seen that for the level 2 dosupply, at a certain RF input power (0.1 normalized) the output powerreaches a maximum saturated output power (1 normalized). The outputpower can be reduced to any value below this saturated maximum (i.e.,“backing off” the output power) by reducing the RF input power. For lowlevels of RF input and output power, there is a nearly linear(proportional) relation between RF input power and RF output power.However, efficiency in this region is relatively low (e.g., below 40%for RF output powers below 0.25 normalized). The highest efficiency isfound in regions of output power at or somewhat below the level at whichoutput power saturates (e.g., efficiency above 70% for output powerabove 0.83 normalized for supply level 2). Increasing RF input powerbeyond the level that saturates the power amplifier, however, actuallyreduces efficiency. This occurs because total input power—do plusRF—increases but output power does not increase (and in some cases candecrease with further increase in RF input power).

The power and efficiency for the lower voltage do supply (level 1) isnow considered. In this case, the maximum saturated output power is muchlower than for the higher-voltage (level 2) dc supply (e.g., reaching amaximum output power of only 0.25 normalized). The output power canagain be adjusted between zero and this lower maximum value by adjustingRF input power (e.g., backing off the output power by reducing thenormalized input power to values below that which saturates the poweramplifier), but higher output powers (above 0.25 normalized) are notobtainable at this supply level. It should be noted that for values ofoutput power that can be reached at this lower supply level, higherefficiency is achieved using the lower dc supply level 1 than for thehigher dc supply level 2, because the power amplifier is operated closerto its saturated value. Thus, for low values of output power, it isgenerally desirable to use a lower supply voltage value, so long as thedesired output power is achievable and the desired level of linearityand controllability of the power amplifier is achievable.

The variation in efficiency with a given dc voltage supply level is amotivation for power amplifier systems such as “Class G,” that switchthe power amplifier do supply among different levels depending on thedesired RF output power level. Selecting from multiple dc supply valuessuch that the power amplifier operates at as high an efficiency aspossible while being able to provide the desired RF output power canyield significant improvements in efficiency over that achieved with asingle supply level.

FIG. 13 includes two IQ plots illustrating the achievable RF outputs interms of the output RF amplitude (i.e., phasor length, or RF voltageamplitude) for two different supply levels. For a given supply level,there is an RF output amplitude (proportional to the square root of RFoutput power) that may be specified as a maximum for that supply level.This maximum amplitude may be that corresponding to the absolute maximumsaturated output power (under complete compression) for that supplylevel (i.e., the saturation point), as illustrated in FIG. 12, or may bea level somewhat below this. One may limit the maximum amplitude andpower to somewhat slightly lower than those for complete saturation tosimplify predistortion of drive signals (for linearization), to accountfor part-to-part variations in absolute maximum power, to place thespecified maximum level in a desirable location on the efficiency vs.output characteristic, or for other reasons. As illustrated in FIG. 13,with the higher supply level, any RF output voltage vector having anamplitude less than or equal to the radius of the circle labeled L2 canbe synthesized. With the lower supply level, one can synthesize any RFoutput voltage vector having an amplitude less than or equal to theradius of the circle labeled L1.

To exploit the availability of multiple supply voltages to achieveincreased efficiency, one may dynamically switch between the two supplylevels. One way to do this is to switch supply levels based on theamplitude of the RF output vector being synthesized at any given time,such that the higher supply level is utilized whenever the desiredoutput amplitude is between L1 and L2, and the lower supply levelwhenever the desired output amplitude is at or below L1. FIG. 14 is anefficiency versus normalized output power characteristic illustrating animplementation of this approach.

In at least one embodiment, the signal to be synthesized is consideredover a longer interval (e.g., a window of multiple digital samples) andlevel switching is managed based on the moving window of data. Forexample, this may be done in a way that ensures that the desiredinstantaneous output amplitude can always be synthesized, but switchdown to the lower supply level only if the desired output signalamplitude will remain at a level at or below L1 for a minimum duration.Amplitude or time hysteresis or other constraints can likewise be putinto level switching decisions. Moreover, while this is illustrated fortwo power supply levels, the approach may be easily extended to anarbitrary number of supply levels. The length of the window used couldbe preselected, or could be dynamically selected based on one or more ofa range of factors including output power levels (short or long term),channel characteristics, data bandwidth, signal statistics, transmitdata, etc. Likewise, one could select the power supply levels based onthe contents of a block of samples at a time, where the block length isfixed or dynamically selected.

An important consideration in the above discussion is that instantaneousefficiency is higher for operating points where the length of thesynthesized vector is close to the maximum achievable amplitude radius.From this perspective, it is desirable to operate from the lowest supplyfor which the desired output can be synthesized. However, as there is anenergy cost to switching between levels (e.g., owing to capacitorcharging loss and switching loss of transistors setting the supply), itmay be beneficial to overall system efficiency to use ahigher-than-necessary supply level to synthesize an output over a shortduration, if the energy cost of switching back and forth between levelsis greater than that saved by using a lower supply level over thatduration. Moreover, there is also a cost to linearity associated withswitching among levels, in terms of noise injected to the output andloss of linearity owing to temporary disturbance of the power amplifiereach time a level transition is made. Even if there is an overallefficiency penalty, one may choose to operate from a higher supply levelthan necessary to synthesize a particular output over a short durationif it reduces the number of supply transitions necessary, in order toimprove linearity. One may also choose not to briefly step up a powersupply level for a short duration, even if the desired instantaneouspower does not get synthesized (possibly briefly clipping the peak ofthe signal), owing to the loss and noise injection penalty of switchingbetween levels. The impact of level transitions on both efficiency andlinearity may thus be considered as factors in selecting what level touse in a particular interval.

Further advantage may be attained in some embodiments by combining powerfrom multiple power amplifiers (see, e.g., system 10 of FIG. 1). In onemultiple power amplifier embodiment, there may be, for example, twopower amplifiers that can each be supplied from two different (nonzero)supply voltages and an isolating power combiner to combine the outputsignals of the amplifiers in an isolating fashion. The voltage vector atthe output port of the power combiner is a weighted sum of the outputvoltage vectors of the two individual power amplifiers. Likewise, a2-way isolating combiner may produce an isolation voltage vector at thecombiner isolation port that is a weighted difference of the twoindividual power amplifiers. The total energy delivered by the poweramplifiers to the combiner is ideally delivered to either the outputport (e.g., to be transmitted) or to the isolation port (e.g., where itmay be dissipated in an isolation resistor, recovered via an energyrecovery system, or transmitted via a second transmission path). Forexample, if the output phasor vectors of the individual PAs are V₁, V₂,the output port of the power combiner may produce a sum vector:Σ=V _(out) =V ₁ +V ₂and the isolation port of the combiner may produce a difference vector:Δ=V _(iso) =V ₁ −V ₂As is known in the art, for an isolating combiner, the input impedancesat the combiner inputs and the impedances of the loads at the output andisolation ports scale such that energy is conserved, as described above.

Because energy delivered to the isolation port is usually lost, it isusually desirable to synthesize the desired output voltage vector whileminimizing the amplitude of the voltage vector at the isolation port.One may thus select the power supply levels, amplitudes and phases ofthe two power amplifiers to deliver the desired output while puttingsmall (and preferably the minimum) power into the isolation port.Alternatively, if the signal to be provided at the isolation port is tobe transmitted by a corresponding antenna rather than being dissipated,it may be desirable to control the supply voltages and PA inputamplitudes such that the desired signals are synthesized at both theoutput and isolation ports while providing maximum overall efficiency ofdelivering energy to the two ports.

FIG. 15 is an IQ plot illustrating achievable output signal ranges for a2-amplifier system where each power amplifier is supplied from one oftwo supply levels. This is illustrated as regions on the IQ diagram interms of the output RF amplitude (phasor length, or RF voltageamplitude). As illustrated, each power amplifier can generate an RFoutput with a maximum amplitude that depends on the power supply levelselected for that amplifier, if both PAs are supplied using a highervoltage L2, an RF output vector anywhere inside the circle labeled L2/L2can be synthesized, with this maximum achieved when the two individualPA vectors are in phase and at their specified maximum amplitudes forthat supply level. If one PA is supplied at the higher voltage level andthe other at the lower voltage level, an RF output vector anywhereinside the circle labeled L2/L1 can be synthesized, with this maximumachieved when the two individual PA vectors are in phase and each attheir specified maximum amplitudes for their supply levels. At thismaximum amplitude, nonzero output is delivered to the isolation port ofthe combiner, since there is a nonzero difference between the PAoutputs. If both PAs are supplied at the lower voltage level, an RFoutput vector anywhere inside the circle labeled L1/L1 can besynthesized. Lastly, if only one PA, supplied at the lower voltage leveldrives the combiner (e.g., with the drive amplitude of the second PA setto zero and/or the power supply for the second PA set to zero), then avector anywhere inside the circle labeled L1/− can be synthesized.Again, there is nonzero output delivered to the isolation port for thiscase if the combiner is isolating. An additional circle with only one PAsupplied at the higher supply voltage level driving the combiner willalso typically exist. However, operation within this circle will usuallybe less efficient than the other options. The above-described techniquesmay be extended for use within systems having more than two PAs, systemshaving more than two nonzero supply levels, systems using other types ofpower combiners, and/or a combination of the above.

FIG. 16 is an IQ plot illustrating operation of an exemplary AsymmetricMulitlevel Outphasing (AMO) based amplification system that includestwo-power amplifiers and two power supply levels. Each of the poweramplifiers may be operated at a specified maximum output amplitude (withrespect to the power supply voltage used by each amplifier) such thatthe efficiency of each PA can be high. To modulate output power, the twoPAs may be outphased (or phase shifted) such that the sum vector has thedesired amplitude and phase. With an isolating combiner, there may besome dissipation associated with the difference vector sent to theisolation port. By changing the power supply levels depending upon thedesired output amplitude, one can keep the amplitude of the differencevector small, maintaining high efficiency.

In at least one embodiment, multiple power amplifiers may be controlledsuch that they synthesize individual output vectors that are in phase,but with amplitudes that may be different (and selecting among differentsupply levels) depending upon desired output. That is, for a givensupply level selection for the two PAs, there is an achievable outputamplitude maximum, which is reached by operating the two PAs at theirachievable maximum. One can reduce the output below this by backing offone or both PAs below their achievable maximum. While backing off thePAs reduces PA efficiency, keeping the PAs in phase keeps the amplitudeat an isolation port small, which can provide enhanced efficiency. Inone approach, the lowest supply level set may be utilized that willenable the desired output to be achieved, and the drive of one or bothPAs will then be backed off to reduce power for output amplitudes abovethat achievable with the next lower supply level set. This keeps the PAsoperated at or close to their achievable maximum while minimizing theoutput at the isolation port. FIG. 17 is an IQ plot illustrating thisapproach. FIG. 18 is a series of plots illustrating how power amplifierdrive levels and supply levels may be adjusted to control output powerover an output power range in accordance with an embodiment. As shown,peaks in efficiency may be attained through judicious supply levelselection as a function of power, and good efficiency may be maintainedover a wide output power range.

In some embodiments, both backoff and outphasing techniques may becombined in a power amplification system to reduce output amplitudebelow an achievable maximum with a given set of supply voltages, asillustrated in FIG. 15. In this case, for each supply level set andoutput amplitude, a combination of backoff and outphasing can beselected to provide desirable tradeoffs between efficiency andlinearity. Moreover, at very low power, one of the power amplifiers maybe completely turned off and the other amplifier may be operated underbackoff alone. It should also be appreciated that the combination ofoutphasing and backoff of the individual power amplifiers can be used toprovide further control features. For example, instead of a differencevector being delivered to an isolation port to be dissipated, the“isolation port” can be connected to a second output for RF transmission(i.e., for a multi-output system). By controlling both backoff andoutphasing of the two amplifiers, desired outputs can be provided atboth the output port (first output) and the isolation port (secondoutput) while preserving high efficiency.

In some embodiments, multiple different types of power amplifiers, orpower amplifiers optimized for different characteristics, may be usedwithin a power amplification system. The parameters of the system maythen be controlled to best utilize the two types or characteristics ofpower amplifiers. For example, it may be desirable to optimize a firstpower amplifier for operation near its achievable maximum (e.g., aswitched-mode or saturated power amplifier) and a second power amplifierfor operation with good efficiency under a range of backoff (e.g., usinga Doherty power amplifier or chireix outphasing power amplifier). Inthis case, one may control the system such that the first poweramplifier usually operates at or near its achievable maximum, while thesecond regulates the output amplitude by backing off from its maximum.Different output power regions can further be covered by appropriatelyswitching the supply levels of the power amplifier, with the secondpower amplifier operated at the higher of the supply levels if thesupply levels are not the same. At the lowest output levels, the firstpower amplifier may be turned off, and the output controlled only withthe second power amplifier.

In some embodiments, as described above, more than two power amplifiersmay be used to generate a transmit signal in an RF transmitter. Forexample, FIG. 3 is a block diagram illustrating an RF transmitter 50that includes at least four power amplifiers in accordance with anembodiment. As illustrated, RF transmitter 50 may include: a controller52; first, second, third, and fourth digital to RF modulators 54, 56,58, 60; first second, third, and fourth power amplifiers 62, 64, 66, 68;a power combiner 70; and a voltage control unit 72. As before, one ormore antennas 32 may be coupled to an output of combiner 70. Voltagecontrol unit 72 may provide variable supply voltages V₁(t), V₂(t),V₃(t), V_(n)(t) to first, second, third, and fourth power amplifiers 62,64, 66, 68, respectively, based on one or more control signals fromcontroller 52. First, second, third, and fourth digital to RF modulators54, 56, 58, 60 provide RF input signals to first, second, third, andfourth power amplifiers 62, 64, 66, 68, respectively based on inputinformation received from controller 52. Combiner 70 combines the outputsignals of first, second, third, and fourth power amplifiers 62, 64, 66,68 to generate an RF transmit signal for delivery to antenna(s) 32.

Controller 52 may use any of the control techniques described above invarious embodiments. In some implementations, controller 52 may use thevoltage control of voltage control unit 72 and the phase and amplitudeinformation delivered to first, second, third, and fourth digital to RFmodulators 54, 56, 58, 60 to ensure that transmit data is accuratelyrepresented within the RF output signal of combiner 70. In addition,controller 52 may use amplitude information delivered to first, second,third, and fourth digital to RF modulators 54, 56, 58, 60 tocontrol/adjust an output power level of combiner 70 (e.g., a transmitpower level, etc.). In some embodiments, this output power controlcapability may be used to provide power backoff for RF transmitter 50.As in previous embodiments, an energy recovery module 30 may be providedto recover energy at an isolation port of combiner 70 when an isolatingcombiner architecture is used.

In at least one embodiment, an RF transmitter may be provided thatincludes a single RF power amplifier. FIG. 4 is a block diagramillustrating an exemplary RF transmitter 80 that includes a single poweramplifier in accordance with an embodiment. As shown, RF transmitter 80includes: a controller 82, a digital to RF modulator 84, a poweramplifier 86, and a voltage control unit 88. The output of poweramplifier 86 may be coupled to one or more antennas 32 to facilitate thetransmission of RF transmit signals to remote wireless entities. Voltagecontrol unit 88 may provide a variable supply voltage V(t) to poweramplifier 86 based on a control signal from controller 82. Digital to RFmodulator 84 may provide an RF input signal to power amplifier 86 basedon input information (e.g., I₁, Q₁) received from controller 82.Controller 82 may use any of the control techniques described above invarious embodiments. In some implementations, controller 82 may use thevoltage control of voltage control unit 88 and the amplitude and phaseinformation delivered to digital to RF modulator 84 to ensure that thetransmit data is accurately represented within the RF output signal ofRF transmitter 80. Controller 82 may use the amplitude informationdelivered to digital to RF modulator 84 to control/adjust an outputpower level of RF transmitter 80 (e.g., a transmit power level). Asbefore, in some implementations, this output power control capabilitymay be used to provide power backoff for RF transmitter 80.

FIG. 5 is a block diagram illustrating an exemplary RF transmitter 100in accordance with an embodiment. RF transmitter 100 is a specificimplementation of RF transmitter 40 of FIG. 2. As illustrated, RFtransmitter 100 includes: a controller 90; first and second digital toRF modulators 92, 94; first and second power amplifiers 96, 98; avoltage control unit 102; a power combiner 104; and an energy recoverymodule 120. In the illustrated implementation, voltage control unit 102includes: a multi-level power converter 106; first and second switchunits 108, 110; first and second supply selects 112, 114; and first andsecond transition shaping filters 116, 118. Multi-level power converter106 is operative for generating a number of different voltage potentials(i.e., V₁, V₂, V₃, V₄) on a plurality of corresponding voltage lines.Although illustrated with four different voltage levels, it should beappreciated that any number of different levels may be used in differentembodiments. First and second switch units 108, 110 are each capable ofcontrollably coupling one of the plurality of voltage lines at a time toa power supply input of a corresponding power amplifier 96, 98. Firstand second supply selects 112, 114 are operative for switching the firstand second switch units 108, 110 between the different voltage lines inresponse to switch control signals S₁(t), S₂(t) received from controller90. In at least one embodiment, multilevel power converter 106 may becapable of adapting the voltage values on the plurality of voltage linesslowly over time to, for example, accommodate slow variations in adesired average output power. First and second transition shapingfilters 116, 118 are operative for shaping the transitions betweenvoltage levels within each power amplifier supply voltage signal. In atleast one implementation, one or both of the transition shaping filters116, 118 may include a low pass filter. In some implementations,transition shaping filtration is not used.

As shown in FIG. 5, controller 90 may provide a time varying amplitudevalue and a time varying phase value to each of the digital to RFmodulators 92, 94. That is, controller 90 may provide A₁(t), θ₁(t) tofirst digital to RF modulator 92 and A₂(t), θ₂(t) to second digital toRF modulator 94. As described previously, in some implementations, theamplitude and phase information may be specified using time varying Iand Q data (or some other format), rather than amplitude and phasevalues.

FIG. 6 is a schematic diagram illustrating an exemplary voltage controlunit 120 in accordance with an embodiment. As illustrated, voltagecontrol unit 120 may include: first and second multi-output (ormulti-level) power converters 122, 124; first and second switch units126, 128; and first and second supply selects 130, 132. Firstmulti-output power converter 122, first switch unit 126, and firstsupply select 130 may be used to vary a supply voltage of a first poweramplifier of an RF transmitter and second multi-level power converter124, second switch unit 128, and second supply select 132 may be used tovary a supply voltage of a second power amplifier of the RF transmitter.First and second switch units 126, 128 and first and second supplyselects 130, 132 may operate in substantially the same manner as thecorresponding units described above.

As illustrated in FIG. 6, first multi-output power converter 122 maygenerate a first plurality of voltage levels (i.e., V₁, V₂, V₃, V₄) on afirst plurality of voltage lines 134 and second multi-output powerconverter 124 may generate a second plurality of voltage levels V₅, V₆,V₇, V₈) on a second plurality of voltage lines 136. In some embodiments,the voltage levels on the first plurality of voltage lines 134 may bethe same as the voltage levels on the second plurality of voltage lines136. In other embodiments, they may be different. As describedpreviously, in some embodiments, different power amplifier designs maybe used for the multiple different power amplifiers within an RFtransmitter. These different power amplifier designs may requiredifferent discrete voltage levels for optimal performance. In theembodiment depicted in FIG. 6, the same number of voltage levels areprovided for the first and second power amplifier. In some otherembodiments, different numbers of voltage levels may be used fordifferent power amplifiers.

In some implementations, first and second multi-output power converters122, 124 may each be capable of adapting corresponding output voltagelevels over time. As described above, this may be performed to, forexample, accommodate slow variations in a desired average output powerlevel. In at least one implementation, first and second multi-levelpower converters 122, 124 may each include a control input 138, 140 toreceive a control signal from a corresponding controller (not shown)indicating when such adaptation is to take place.

FIG. 7 is a schematic diagram illustrating an exemplary voltage controlunit 160 in accordance with an embodiment. As illustrated, voltagecontrol unit 160 may include: first, second, third, and fourth powersupplies 162, 164, 168, 170; first and second switch units 170, 172; andfirst and second supply selects 174, 176. First, second, third, andfourth power supplies 162, 164, 168, 170 may each be operative forgenerating a unique supply potential on a corresponding output line.First and second switch units 170, 172 may each be capable ofcontrollably coupling one of the supply potentials at a time to a powersupply input of a corresponding power amplifier. First and second supplyselects 174, 176 are operative for switching the first and second switchunits 170, 172 between the different supply potentials in response toswitch control signals S₁(t), S₂(t) received from a correspondingcontroller. In some implementations, first, second, third, and fourthpower supplies 162, 164, 168, 170 may each adapt its output voltagevalue slowly over time to, for example, accommodate slow variations indesired average output power level. In at least one embodiment, first,second, third, and fourth power supplies 162, 164, 168, 170 eachcomprise a battery. Although illustrated with four power supplies 162,164, 168, 170, it should be appreciated that any number of differentpower supplies may be used in a particular implementation.

FIGS. 8 and 9 are flow diagrams illustrating processes for operating RFtransmitters in accordance with various embodiments.

The rectangular elements (typified by element 182 in FIG. 8) are hereindenoted “processing blocks” and may represent computer softwareinstructions or groups of instructions. It should be noted that the flowdiagrams of FIGS. 8 and 9 represent exemplary embodiments of designsdescribed herein and variations in such diagrams, which generally followthe processes outlined, are considered to be within the scope of theconcepts, systems, and techniques described and claimed herein.

Alternatively, the processing blocks may represent operations performedby functionally equivalent circuits, such as a digital signal processorcircuit, an application specific integrated circuit (ASIC), a fieldprogrammable gate array (FPGA), or conventional electrical or electronicsystems or circuits. Some processing blocks may be manually performed,while other processing blocks may be performed by circuitry and/or oneor more processors. The flow diagrams do not depict the syntax of anyparticular programming language. Rather, the flow diagrams illustratethe functional information one of ordinary skill in the art may requireto fabricate circuits and/or to generate computer software or firmwareto perform the corresponding processing. It should be noted that manyroutine program elements, such as initialization of loops and variablesand the use of temporary variables, are not shown. It will beappreciated by those of ordinary skill in the art that unless otherwiseindicated herein, the particular sequences described are illustrativeonly and can be varied without departing from the spirit of the conceptsdescribed and/or claimed herein. Thus, unless otherwise stated, theprocesses described below are unordered meaning that, when possible, thesequences shown in FIGS. 8 and 9 can be performed in any convenient ordesirable order.

Referring now to FIG. 8, a method 180 for operating an RF transmitterhaving at least two digital-to-RF modulators driving at least two poweramplifiers will be described. First, transmit data may be obtained thatis intended for transmission to one or more remote wireless entities(block 182). The transmit data may be used to provide input informationfor the at least two digital-to-RF modulators to control amplitudes andphases of output signals of the modulators (block 184). The inputinformation generated for the at least two digital-to-RF modulators maybe different for different modulators. The output signals of the atleast two digital-to-RF modulators may then be delivered tocorresponding inputs of the at least two power amplifiers of the RFtransmitter (block 186).

The transmit data may also be used to select supply voltages for each ofthe at least two power amplifiers (block 188). The output signals of theat least two power amplifiers may be combined to generate a transmitteroutput signal for the RF transmitter (block 190). Any of the controltechniques described herein may be used to generate the inputinformation for the at least two digital-to-RF modulators and to selectthe supply voltages for the at least two power amplifiers. Thetransmitter output signal may be delivered to one or more antennas to betransmitted into a wireless channel (block 192). In at least oneimplementation, the input information generated for each digital-to-RFmodulator may be representative of both an amplitude value and a phasevalue. The amplitude value and the phase value will typically vary withtime (i.e., they can change from sample to sample). In at least oneimplementation, I and Q values may be provided for each of the at leasttwo digital-to-RF modulators, in some other implementations, timevarying amplitude and phase values may be provided. As described above,different input information may be generated for each of the at leasttwo digital-to-RF modulators.

In at least one approach, the amplitude and phase values provided forthe at least two digital-to-RF modulators and the supply voltagesselected for the at least two power amplifiers may be selected in amanner that results in an accurate representation of the transmit datawithin the RF output signal of the RF transmitter. The amplitude valuesgenerated for the at least two digital-to-RF modulators may be selectedto control or adjust an output power level of the RF transmitter in someembodiments. In one approach, the amplitude values may be selected toachieve a desired level of power backoff within the RF transmitter.

FIG. 9 is a flow diagram illustrating a method 200 for operating an RFtransmitter having a single digital-to-RF modulator and a single poweramplifier. Transmit data is first obtained that is intended fortransmission to one or more remote wireless entities (block 202). Thetransmit data may be used to generate input information for thedigital-to-RF modulator to control an amplitude and phase of an outputsignal of the modulator (block 204). The amplitude of the output signalof the modulator may be controlled in order to achieve a desiredtransmit power level for the RF transmitter. The output signal of thedigital-to-RF modulator may be delivered to a corresponding input of thepower amplifier of the RF transmitter (block 206).

The transmit data may also be used to select a supply voltage for thepower amplifier (block 208). Any of the control techniques describedherein may be used to generate the input information for the singledigital-to-RF modulator and to select the supply voltage for the singlepower amplifier. The output signal of the power amplifier may be coupledto one or more antennas for transmission into a wireless channel (block210). In at least one implementation, the input information delivered tothe digital-to-RF modulator may be representative of both an amplitudevalue and a phase value. In at least one approach, the phase valuegenerated for the digital-to-RF modulator and the supply voltageselected for the power amplifier may be selected in a manner thatprovides an accurate representation of the transmit data at an output ofthe RF transmitter. The amplitude value generated for the digital-to-RFmodulator may be selected to control or adjust an output power level ofthe RF transmitter. In one approach, this power control capability maybe used to achieve a desired level of power backoff for the RFtransmitter.

FIG. 11 is a flow diagram illustrating a method 230 for selectingvoltage levels for one or more power amplifiers of a power amplificationsystem based on a window of samples in accordance with an embodiment. Asdescribed previously, FIG. 10 is a diagram illustrating such a samplewindow 220. The method 230 may be used, for example, within the methods180, 200 of FIGS. 8 and 9 (e.g., in block 188 and block 208,respectively). As shown in FIG. 11, it may first be determined whetherN_(W) samples can use a lower set of voltage levels than a previoussample and still meet output power requirements (block 232). If so(block 232—Y), then a new set of voltage levels may be selected that areas low as possible (or at least lower than the previous sample) tosupport all N_(W) samples (block 234). If the N_(W) samples cannot use alower set of voltage levels than the previous sample, it may next bedetermined whether the set of voltage levels used for the previoussample is sufficient to support the current sample (block 236). If not(block 236—N), then a new set of voltage levels may be selected that areas low as possible to support the current sample (block 238). If the setof voltage levels used for the previous sample is sufficient to supportthe current sample (block 236—Y), then the same level selection may beused for the current sample that was used for the previous sample. Thisprocess may be repeated for each new sample.

In the description above, various concepts, circuits, and techniques arediscussed in the context of RF transmitters that are operative fortransmitting signals via a wireless medium. It should be appreciatedthat these concepts, circuits, and techniques also have application inother contexts. For example, in some implementations, features describedherein may be implemented within transmitters or drivers for use inwireline communication. In some other implementations, featuresdescribed herein may be implemented within other types of systems thatrequire highly efficient and highly linear power amplification for datacarrying signals.

Having described exemplary embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may also be used. Theembodiments contained herein should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A radio frequency (RF) transmitter, comprising: afirst RF power amplifier; a second RF power amplifier; a combiner tocombine output signals of the first and second RF power amplifiers; afirst digital-to-RF modulator to generate an RF input signal for thefirst RF power amplifier based on first information indicative of afirst amplitude and a first phase; a second digital-to-RF modulator togenerate an RF input signal for the second RF power amplifier based onsecond information indicative of a second amplitude and a second phase;a first voltage control unit to provide a variable supply voltage to thefirst RF power amplifier in response to a first control signal; a secondvoltage control unit to provide a variable supply voltage to the secondRF power amplifier in response to a second control signal; and acontroller configured to: receive data indicative of a vector to betransmitted from the transmitter; select first and second voltage levelsfor the first and second power amplifiers, respectively, that arecapable of providing the vector at the output of the combiner; selectfirst and second input phase angles for the first and second poweramplifiers, respectively, to achieve the vector at the output of thecombiner; select first and second input amplitude levels to be providedto the first and second power amplifiers, respectively, to achieve thevector at the output of the combiner, wherein at least one of the firstinput amplitude level and the second input amplitude level are such thatthe corresponding power amplifier is backed off from a saturation pointassociated with the voltage level selected for that power amplifier; andgenerate the first information based on the first input phase angle andthe first input amplitude level, generate the second information basedon the second input phase angle and the second input amplitude level,generate the first control signal based on the first selected voltagelevel, and generate the second control signal based on the secondselected voltage level, thereby setting an output power level of the RFtransmitter.
 2. The RF transmitter of claim 1, wherein: the controllerfurther controls the first phase associated with the first informationand the second phase associated with the second information to set anoutput power of the RF transmitter.
 3. The RF transmitter of claim 1,further comprising: at least one additional RF power amplifier, whereinthe combiner combines output signals of the first, the second, and theat least one additional RF power amplifier.
 4. The RF transmitter ofclaim 3, wherein: the at least one additional RF power amplifierincludes a third RF power amplifier; and the RF transmitter furthercomprises: a third digital-to-RF modulator to generate an RF inputsignal for the third RF power amplifier based on third informationindicative of a third amplitude and a third phase; and a third voltagecontrol unit to provide a variable supply voltage to the third RF poweramplifier in response to a third control signal; wherein the controlleris configured to provide the third information and the third controlsignal based, at least in part, on the transmit data.
 5. The RFtransmitter of claim 1, wherein: the first voltage control unit isconfigured to select one of a plurality of available voltages for thefirst RF power amplifier in response to the first control signal.
 6. TheRF transmitter of claim 5, wherein: the controller is configured toselect a lowest set of voltage supply levels for the first and second RFpower amplifiers that are capable of achieving a desired instantaneousoutput power level of the RF transmitter.
 7. The RF transmitter of claim5, wherein: the controller provides a substantially fixed phaserelationship between the output of the first power amplifier and theoutput of the second power amplifier.
 8. The RF transmitter of claim 1,wherein: the controller is configured to select the first and secondinput phase angles to produce individual output vectors of the first andsecond power amplifiers that are in phase and that produce the phaseangle of the vector at the output of the combiner.
 9. The RF transmitterof claim 1, wherein: the first voltage control unit is configured toselect one of a plurality of available voltages for the first RF poweramplifier in response to the first control signal and the second voltagecontrol unit is configured to select one of the plurality of availablevoltages for the second RF power amplifier in response to the secondcontrol signal; and the first and second voltage control units arecapable of adapting voltage values of the plurality of availablevoltages over time.
 10. The RF transmitter of claim 5, wherein: thefirst voltage control unit includes a first transition shaping filter toshape transitions between voltage levels within an output signal of thefirst voltage control unit.
 11. The RF transmitter of claim 5, wherein:the power combiner is an isolating combiner having an isolation port;and the RF transmitter further comprises an energy recovery circuitcoupled to the isolation port of the isolating combiner to recoverenergy output from the isolation port and convert the recovered energyto a useable form.
 12. The RF transmitter of claim 5, wherein: the firstand second RF power amplifiers have different amplifier architectures.13. The RF transmitter of claim 12, wherein: one of the first and secondpower amplifiers is a Doherty amplifier and the other is not.
 14. The RFtransmitter of claim 5, wherein: the first and second RF poweramplifiers have different amplifier designs.
 15. The RF transmitter ofclaim 5, wherein: the controller is configured to update the firstinformation, the second information, the first control signal, and thesecond control signal on a sample by sample basis.
 16. The RFtransmitter of claim 1, wherein: the first voltage control unit isconfigured to select one of a plurality of available voltages for thefirst RF power amplifier in response to the first control signal and thesecond voltage control unit is configured to select one of a pluralityof available voltages for the second RF power amplifier in response tothe second control signal; and the controller is configured to makedecisions about voltage level changes for the first and second poweramplifiers based, at least in part, on a window of samples representingdata to be transmitted by the transmitter.
 17. The RF transmitter ofclaim 16, wherein: the controller is configured to adapt a length of thewindow of samples used to make voltage level change decisions over time.18. The RF transmitter of claim 1, wherein: the first voltage controlunit is configured to select one of a plurality of available voltagesfor the first RF power amplifier in response to the first control signaland the second voltage control unit is configured to select one of aplurality of available voltages for the second RF power amplifier inresponse to the second control signal; and to determine voltage levelsfor the first and second power amplifiers, the controller is configuredto: identify a window of samples to be transmitted by the transmitter;if all samples in the window of samples can use a lower set of voltagelevels than a previous sample to achieve a desired output power, selecta new set of voltage levels that are lower than those used for theprevious sample to support all samples in the window; if all samples inthe window cannot use a lower set of voltage levels than the previoussample to achieve the desired output power: if the set of voltage levelsused for the previous sample is sufficient for use with a currentsample, use the set of voltage levels used for the previous sample forthe current sample; and if the set of voltage levels used for theprevious sample is not sufficient for use with the current sample,select a new set of voltage levels for the current sample.
 19. The RFtransmitter of claim 5, wherein: the combiner is an isolating combinerhaving a sum port and a difference port; and the controller isconfigured to provide the first information, the second information, thefirst control signal, and the second control signal to minimize anoutput power level at the difference port of the combiner.
 20. A machineimplemented method for operating an RF transmitter having at least twodigital-to-radio frequency (RF) modulators driving at least two poweramplifiers and a combiner to combine output signals of the at least twopower amplifiers, the method comprising: obtaining transmit data to betransmitted from the RF transmitter; generating input information forthe at least two digital-to-RF modulators based, at least in part, onthe transmit data, the input information to control amplitudes andphases of RF output signals of the at least two digital-to-RFmodulators, wherein generating input information includes controllingone or more amplitudes associated with the at least two digital-to-RFmodulators to set an output power level of the RF transmitter;delivering the output signals of the at least two digital-to-RFmodulators to corresponding inputs of the at least two power amplifiers;selecting supply voltages for each of the at least two power amplifiersbased, at least in part, on the transmit data by the steps of: selectingsupply voltage levels for each of the at least two power amplifiers, thesupply voltage levels capable of providing a vector at the output of thecombiner to be transmitted from the transmitter; selecting input phaseangles for each of the at least two power amplifiers to achieve thevector at the output of the combiner; selecting and providing inputamplitude levels to each of the at least two power amplifiers to achievethe vector at the output of the combiner, wherein at least one of theinput amplitude levels are such that the corresponding power amplifieris backed off from a saturation point associated with the voltage levelselected for that power amplifier; and generating the input informationbased on the selected input phase angles, the selected input amplitudelevels and the selected supply voltage for each corresponding poweramplifier, thereby setting an output power level of the RF transmitter;and combining the output signals of the at least two power amplifiers;and generating a transmitter output signal at the set output powerlevel.
 21. The method of claim 20, wherein: selecting supply voltagesfor each of the at least two power amplifiers includes selecting alowest set of supply levels for the at least two power amplifiers thatare capable of achieving a desired instantaneous output power level ofthe RF transmitter.
 22. The method of claim 20, wherein: generatinginput information for the at least two digital-to-RF modulators includesgenerating information that provides a substantially fixed phaserelationship between outputs of the at least two power amplifiers. 23.The method of claim 20, wherein: combining the output signals of the atleast two power amplifiers includes applying the output signals of theat least two power amplifiers to an isolating combiner having anisolation port; and the method further comprises recovering energyoutput from the isolation port of the isolating combiner and convertingthe recovered energy to a useable form.
 24. The method of claim 20,wherein: selecting supply voltages for each of the at least two poweramplifiers includes selecting the supply voltages based, at least inpart, on a window of samples representing data to be transmitted by thetransmitter.
 25. The method of claim 20, wherein selecting supplyvoltages for each of the at least two power amplifiers comprises:identifying a window of samples to be transmitted; if all samples in thewindow of samples can use a lower set of voltage levels than a previoussample to achieve a desired output power, selecting a new set of voltagelevels that are lower than those used for the previous sample to supportall samples in the window; if all samples in the window cannot use alower set of voltage levels than the previous sample to achieve thedesired output power: if the set of voltage levels used for the previoussample is sufficient for use with a current sample, using the set ofvoltage levels used for the previous sample for the current sample; andif the set of voltage levels used for the previous sample is notsufficient for use with the current sample, selecting a new set ofvoltage levels for the current sample.
 26. The method of claim 25,wherein: selecting a new set of voltage levels that are lower than thoseused for the previous sample to support all samples in the windowincludes selecting a new set of voltage levels that are as low aspossible while still being able to provide a desired instantaneousoutput power for all samples.
 27. The method of claim 25, wherein:selecting a new set of voltage levels for the current sample includesselecting a new set of voltage levels that are as low as possible foruse with the current sample.
 28. An apparatus comprising anon-transitory computer readable medium having instructions storedthereon that, when executed by a computer, perform a method foroperating an RF transmitter having at least two digital-to-radiofrequency (RF) modulators feeding at least two power amplifiers and acombiner to combine output signals of the at least two power amplifiers,the method comprising: obtaining transmit data to be transmitted fromthe RF transmitter; providing input information for the at least twodigital-to-RF modulators based, at least in part, on the transmit data,the input information to control amplitudes and phases of RF outputsignals of the at least two digital-to-RF modulators, wherein generatinginput information includes controlling one or more amplitudes associatedwith the at least two digital-to-RF modulators to set an output powerlevel of the RF transmitter; and selecting supply voltages for each ofthe at least two power amplifiers based, at least in part, on thetransmit data by: selecting supply voltage levels for each of the atleast two power amplifiers, the supply voltage levels capable ofproviding a vector at the output of the combiner to be transmitted fromthe transmitter; selecting input phase angles for each of the at leasttwo power amplifiers to achieve the vector at the output of thecombiner; selecting and providing input amplitude levels to each of theat least two power amplifiers to achieve the vector at the output of thecombiner, wherein at least one of the input amplitude levels are suchthat the corresponding power amplifier is backed off from a saturationpoint associated with the voltage level selected for that poweramplifier; and generating the input information based on the selectedinput phase angles, the selected input amplitude levels and the selectedsupply voltage for each corresponding power amplifier, thereby settingan output power level of the RF transmitter.
 29. The apparatus of claim28, wherein: selecting supply voltages for each of the at least twopower amplifiers includes selecting a lowest set of supply voltages forthe at least two power amplifiers that are capable of achieving adesired instantaneous output power level of the RF transmitter.
 30. Theapparatus of claim 28, wherein: selecting supply voltages for each ofthe at least two power amplifiers includes selecting the supply voltagesbased, at least in part, on a window of samples representing data to betransmitted by the transmitter.